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(Meta) On the Natural Biology of Centaurs

This little essay was inspired partly by my recent fic 'Majestic Creatures of Legend', partly by peasant_'s fascinating essays on the biology of vampirism, and partly by the fact that I'm totally insane.

Yes, after several hours spent following links on Wikipedia and various websites giving advice on raising and looking after horses, I give you a serious, academic treatise on the natural biology of... the centaur. You know, like Dawn turns into in Season 8, although most of what I say should be applicable to centaurs from other fantasy and mythological sources.

If you're squicked by the use of terms such as 'digestive tract', 'oestrus' or 'trachea', you may not want to read this. If you're a qualified veterinarian, horse-breeder, doctor, or indeed a majestic creature of legend yourself and spot any errors, please feel free to correct me. The rest of you... I advise you to shake your heads sadly and move on quickly to the next post in your friends list. That'll be better for all of us in the long run.

On the Natural Biology of Centaurs

Introduction

A centaur is a creature made up of the upper body of a human merged with the lower body of a horse. The human torso extends about as far as the navel - and the pictures of Dawn in S8 show that she does indeed have a navel there, which raises some interesting questions about centaur gestation. The horse body extends from the base of its neck downwards.

Obviously such a creature is magical in origin, and its bodily functions may depend to a greater or lesser degree on magic to even operate. However, for the purpose of this essay I will assume that where both a biological and a purely magical 'handwave' solution exist for the same problem, the biological one will be preferred.

On first glance, a centaur's organs can be divided neatly into 'human' and 'equine' categories. However, closer examination shows that this cannot be entirely the case. A centaur only has one brain, in its human head; but this must control the autonomic functions of the horse body as well. A centaur has to coordinate six limbs - two arms and four legs - not just four. In short, a centaur's brain is not identical to a human brain. My assumption will therefore be that for the most part the human and equine organs operate as they would in a person or a horse, respectively, but where necessary there will be unique centaur features.

Circulatory and Nervous Systems

Like Timelords, centaurs have two hearts. I assume the blood circulates throughout the entire body, rather than having separate circulatory systems for each section.

As described above, the centaur's nervous system is controlled by a single brain, in the human section. However, the spinal column extends into the horse section, and I assume the nerves are integrated. Dawn experiences the sensation of pulling her mane in the same way as she does pulling her hair. We can assume that other forms of touch will work the same way regardless of whether the area contacted is human or equine - making allowance for the fact that in general a horse's hide is thicker and less sensitive than a human's skin.

Respiration

Respiration poses more of a problem. The human lungs work the same way as those of a normal human. However, there is no obvious way for air to reach the equine lungs. There are three potential solutions to this problem:

1. The equine lungs are non-functional. This seems unsatisfactory, because human lungs alone seem insufficient to provide the amount of oxygen needed by a full-size centaur body.

2. The air reaches the lungs by magical teleportation. This would work, but violates the principle of preferring biological to magical solutions.

3. A centaur's trachea extends down through the human torso, past the liver and stomach, and into the horse torso. Presumably it branches part way down, with one fork going into the human lungs and the other going further down into the horse ones. This seems the best biological solution, although it does mean that the internal organs of a centaur's human torso differ significantly from those of a normal human. Still, we've already accepted that principle when it comes to the brain.

Assuming the third solution is accepted, what this would mean for Dawn is that she'd experience a much greater volume of air passing through her air passages whenever she inhaled or exhaled: it would feel like panting or yawning even when she was breathing normally. I suspect she might need to breathe through her mouth as well as her nose, leading to symptoms of snoring and morning dry-mouth.

Digestion and Diet

A similar problem confronts us when it comes to digestion. The human section has a mouth and oesophagus, a stomach, and probably some of the intestines, but not room for all of them; it has no rectum or anus. The horse section has no mouth or oesophagus, but all the other parts (including some duplicates). Complicating the matter is that humans are omnivores and able to digest meat; horses cannot derive nutrition from meat, but can digest cellulose from plant fibres allowing them to eat grass and hay. We know from Season 8 that Dawn has a craving for hay, so presumably her equine digestive system is able to handle it. Chewing and swallowing would pose greater problems for a human mouth, though I dare say Dawn is resourceful enough to prepare and flavour the hay to make it more palatable. Even as a human she was known for her culinary adventurousness...

As with respiration, the question is: do we posit that the hay teleports from her mouth to her equine stomach; or do we assume a physical connection that it can travel through? If we assume the second, we again have two options. Under one, her oesophagus forks like her trachea, and some food goes into her human stomach while the rest is transported down into her horse stomach. That would require a complex system to determine which stomach the food goes into, either under conscious control or influenced by the taste and smell of the food triggering the appropriate reaction in the brain. There would also need to be a second connection from out of the human stomach bypassing the horse's and feeding into its intestines.

The simpler approach is that the human and horse stomachs are simply connected in series. Food that a human can digest is processed in the human stomach; otherwise it passes straight through there so that the horse's caecum can break down the cellulose. The process would presumably be fairly transparent to the centaur, although she might be more prone to stomach ache and colic than normal. Also, the digestive process as a whole would take longer than it would in either a human or a horse because the food has further to travel.

"Eating like a horse" is a proverbial expression, and because a centaur's body mass is greater, they presumably have to eat even more than that. However, a couple of factors balance this. A centaur's omnivorous digestive system is able to process food with a much higher nutritional value than the grain that makes up a horse's diet, so they need less of it. Secondly, one reason horses eat so frequently is because their stomach is small relative to their size; the addition of a centaur's human stomach also helps to counterbalance this.

In other words, Dawn will probably feel hungry most of the time and feel she's eating like a pig, but she won't actually be eating as much by volume as a horse would. (And certainly less than she did as a giant - but it would probably feel like more because her mouth and throat are now human-sized.)

A centaur would need to consume almost as much liquid per day as a human and a horse combined, due to their body mass. Horses drink between twenty and forty times as much as humans do (40 litres per day) so Dawn will feel constantly thirsty and should rarely be seen without a litre bottle of water in her hand. Of course, all that liquid needs to come out again... something we've also seen mentioned in canon. *g* A centaur's human section cuts off above the point where their kidneys and bladder would be, so these functions have to be handled by the equine organs alone. Assuming the bladder is the same size as a normal horse's, a centaur would therefore need to urinate slightly more often than a horse - perhaps 5% more frequently.

Reproduction

The human torso of a centaur also cuts off above the point where their reproductive organs would be, so these functions are also entirely handled by the horse section. This implies that a centaur's breeding patterns and reproductive cycle are closer to those of a horse; but because their brain is mostly human their behaviour and mating strategies will resemble those of humans.

A female horse goes through an oestrual cycle which repeats every three weeks, as compared to the human 28 day menstrual cycle. Unlike humans the lining of the womb is re-absorbed if fertilisation does not occur, rather than being shed - in other words, centaurs do not get periods. (Since a centaur's arms are too short to reach their genitals, I'm betting this was something Dawn was very glad to discover.)

Another difference from humans is that mares are completely unreceptive sexually for two out of every three weeks: presumably this would be experienced by a sentient being as a total loss of libido and lack of interest in sex. At the end of that time, mares go into heat; a state which is characterised by increased bloodflow to the genitals inducing swelling, increased vaginal lubrication, and other symptoms. These continue for 5 - 7 days. Translating that into human terms, a female centaur is going to feel constantly horny from morning until night for an entire week.

If we assume centaurs are a naturally reproducing species, then presumably one centaur must mate with another centaur to produce a baby centaur. Cross-species mating is a more awkward question. My assumption is that the average centaur would feel as repulsed by the idea of bestiality as a human would be - defining it as "sex with a non-intelligent animal". However, in a centaur's eyes a human and a sentient horse-like being such as a unicorn would presumably be seen on equal terms as potential partners. No offspring could result from such a mating, however. Also, phrasing this as delicately as possible, size would definitely be an issue if it came to PIV sex.

(Dawn's case is different since she used to be human and still has human sensibilities - which explains why she reacts with horror when a unicorn tries to get over-friendly with her. However, since this is a woman who feels quite comfortable going out with a three-eyed Thricewise tentacle monster, I suspect she'd easily adapt if that unicorn turned out to be a nice guy and a great conversationalist after all, once she got to know him.)

When female horses are approached by a stallion, they signal their interest in mating by moving their tail aside and then urinating; when a mare is on heat her urine takes on a distinctive smell, and this attracts the stallion to come over and mount her. Whether centaurs also follow this pattern of mating behaviour is unknown; I suspect that even if natural-born centaurs do take it for granted a human turned into a centaur would be horrified and embarrassed by the very idea.

Horses are known to masturbate by rubbing their genitals against things, so it's reasonable to assume that centaurs can do this too. As previously noted, their arms are too short to reach that area directly. For the record, a female horse does have a clitoris (according to Wikipedia) so presumably is capable of reaching orgasm.

Pregnancy, Birth and Nursing

A horse's pregnancy normally lasts eleven months, followed by a short labour giving birth to a foal which is capable of walking around within hours of birth. A centaur's uterus is presumably the same size as a horse's, and yet a centaur foetus will be much larger than an equine one at the same stage of development because it also has its human torso. Putting these facts together suggests that centaur pregnancies may be shorter than those of a horse - perhaps ten months - but labour will be more difficult. The newborn centaur will also be less developed: it may even be as helpless as a human baby.

A centaur's human torso has a navel; assuming its horse torso does too that implies that an embryonic centaur has two umbilical cords connecting to two separate placentas. In addition, female centaurs have four mammary glands - two human ones and two equine ones (the latter tucked up between the hind legs). Presumably they nurse their baby from all of them, although the equine ones will be less useful until the baby can stand in its own four legs or if another adult centaur supports the baby in place.

Physical Capabilities

A horse is able to lock its leg joints in place by bending its hips in a particular way. This allows it to completely relax its muscles and even sleep whilst standing up. Presumably a centaur can do the same thing; remaining in a standing position indefinitely will not be tiring for them. Note, however, that even a horse must lie down occasionally for a deeper sleep.

A horse can gallop at up to 65 kph (40 mph) - some specially bred racehorses are even faster in short bursts. Perhaps not by coincidence, this is also the speed that a Slayer can run at (according to the shooting script of 'Two To Go'). Over a longer distance, speeds of around 15 kph (10 mph) can be sustained for an hour or two. However, horses (and centaurs) are no faster than humans on very long distance treks. Also, note that while a centaur can easily outrun a human, it takes a while for it to coordinate its limbs and get up to speed. Over a very short distance, a human would actually be faster.

A horse requires only two and a half hours of sleep per day, compared to a human's 7 - 8 hours. This sleep is usually obtained in short bursts rather than a continuous period of rest - an adaptation suitable to a herbivorous prey animal. However, I assume that a centaur will require closer to the human level of sleep, since their brains are mostly human - not to mention that they are intelligent omnivores with opposable thumbs and therefore much less vulnerable to sudden attack.

Centaurs appear to be magical creatures rather than demons, assuming such a distinction is valid in the Buffyverse. They almost certainly have souls. Whether they can be turned into vampires is unknown.

So. Any questions? :-)

Comments

This may sound like an unintuitive solution, since it involves the biology of non-mammalian organisms. However, I've been considering the possibility that centaurs might have a respiratory system more similar to that of a bird.

Birds have relatively mammal-like lungs, but in addition they have a series of connected air sacs which increase the volume of the system. The sacs are also linked up in such a way as to maintain a unidirectional flow of air, increasing the efficiency of oxygen transport.

A very large animal like a centaur is going to require a large amount of oxygen, so both additional volume and efficiency are important. Presumably they have no outright avian organs, but if they have both human and equine lungs, it could be that the latter are connected to the former in a similar manner as a bird's air sacs. This would help ease any difficulties a centaur might experience in respiration--the volume of air actually passing through the trachea at any time could be lowered somewhat without impairing oxygenation.

IIRC Philip Jose Farmer created a race of centaurs for his World of Tiers series, and he postulated a number of anatomical adjustments - most of the space in the human torso was taken up by a sac or bellows organ which pumped air down to the lungs proper, which were in the horse torso. I think they were omnivorous, but they also had far larger jaws, wider mouths, and heavier necks than a human would, to accommodate the larger volume of food they had to consume. (And they didn't look like they were likely to snap in two at the waist in a high wind.)

I'm actually a long time fan of centaurs and other human-animal combinations, so I saw a lot of this kind of treatise before. (Did you know there's an active centaur fandom out there?)

A few niggles:

- A centaur isn't that much bigger than a horse, the human upper body is roughly comparable to the neck and head of a horse in size and mass (especially those of a toddler or child). There should be no pregnancy issues.

- Horses can and do occasionally eat meat. They just don't catch it. :) An omnivorous diet might thus alleviate the food amounts a centaur needs to consume.

- With both the lungs and the stomach, I'd assume the main organs to be in the equine torso. The human torso needs only efficient delivery systems. Which would make room for more muscle mass and upper body strngth, which would then explain centaurs beng stronger than humans even though the human body parts are of equivalent size.

Now I need to write a real centaur!Dawn story after that little PWP. Dammit I have enough on my plate already.

Addendum: Meat is probably even be necessary to keep up a sufficient nutrient supply for the brain. Otherwise the body is far too busy digesting to keep up with the demands of a human brain, which needs a considerable part of the nutrients a human takes in.

Horses are known to masturbate by rubbing their genitals against things, so it's reasonable to assume that centaurs can do this too. As previously noted, their arms are too short to reach that area directly. For the record, a female horse does have a clitoris (according to Wikipedia) so presumably is capable of reaching orgasm.

I am now educated about equine masturbatory practices, and I feel my life is better for it. Thank you.

You're welcome. :-) Though would it surprise you to learn that the websites I looked at* had entire paragraphs on male equine masturbation then added at the end "Oh yeah, we think that the females do it too."?

It occurred to me to ask...though I would normally not even think about the subject...how long would a horse or centaur stay satisfied while in heat? I've had family dogs, and I believe I read that once impregnated the urge goes away, but aside from that, I have no idea.

It just occurs to me that if she's only going to stay satisfied, say, a couple of hours or less, Dawn is in for a very long week. Or a short and busy one, possibly, if she can find something suitable, but if not....

I wonder if the young would be born capable of walking and running in a few hours, like a horse and for the same reasons, but then takes years for their human cognitive functions to develop. So they would be very foal like at first, but then gradually grow up to develop speech and higher reasoning.

And now I have a mental picture of a baby centaur gambolling about on spindly legs, making meaningless babbling sounds and clutching a teddy bear to its chest, and need to go wash my brain out before I develop diabetes.

My dad was a vet but he always hated horses (or their owners I'm not sure which came first). I'm just a biologist although more specifically a geneticist. I would have gone with the idea of the human torso part of a centaur being a glorified neck with arms attached but also have some quibbles about diet and reproductive issues.

I think horses can get nutrition from meat or rather meat products They don't have the teeth for slicing up flesh but can be given blood meal in small amounts although they really hate the taste (TMI aside - equine tastes are very different from human, Dad used to try giving us horse Benilyn for coughs, I never had a day off sick from school for the fear of it). More than a small amount and they (the horses) would also have problems with toxic waste products from microbial action in the caecum - colic can be lethal. In evolutionary terms horses are adapted to a near continuous diet of grass but in practise the domesticated ones can cope with two meals of concentrate a day. I imagine centaurs would have problems bending down to eat the grass horse-style, the orientation of the face is all wrong. Also their mouths would be too small to process the volume required and there might be problems with teeth wearing down - horse teeth are big.

The human torso of a centaur also cuts off above the point where their reproductive organs would be, so these functions are also entirely handled by the horse section. The human torso of a centaur also cuts off above the point where their reproductive organs would be, so these functions are also entirely handled by the horse section.Endocrinologically, at least half of the reproductive system is in the brain, the oestrous cycle is controlled by signals between the ovaries, pituitary and hypothalamous (I think the pineal may be involved too but only for circadianicity). I'm so not a physiologist but I suspect that the pattern of hormone release from horse and human ovaries is similar, what differs is how the brain responds to those signals. For this reason I think a centaur's sexuality would be expressed in more human than horse-like ways.

It does seem that a surprising number of supposedly herbivorous animals can, in fact, eat meat if they're fed it. See also, BSE. Excellent point about centaurs not being designed to graze easily: their hands seem more suitable for gathering food from trees and bushes or, indeed, hunting.

I suspect that the pattern of hormone release from horse and human ovaries is similar, what differs is how the brain responds to those signals

Interesting. I think I sort of touched on that by saying that a centaur's brain would need to be different to a human's in order to control the equine organs appropriately, but I didn't realise that for this kind of thing, it was all in the brain.

Even so, does that mean that if the centaur's brain was telling the equine uterus, "No, wait another 7 days... now shed the endometrium! No, don't reabsorb it, shed it!!" the uterus might get a little bit confused? :-)

Ha. I did write a story in which centaur!Dawn lamented her inability to... reach back there for recreational purposes.

I think her biggest problems as a centaur would be a) learning to walk with four legs, and b) consuming enough food and water to sustain her horse physiology.

When she first changed into a centaur she seemed to be already pretty steady on her feet, and we never saw her eating or drinking all that much. However, the same has been true on every occasion that I've seen centaurs depicted in fantasy fiction. It's just a "magical handwave" matter, I guess.

Assuming her brain became rewired to be a centaur brain, I can't see her having too many problems with coordination - although testing her limits and some ingrained cultural reactions might be a problem.

Eating and driknking - I can see her being surprised by how she was thirsty all the time, but I think I can forgive the artists for not showing her stuffing her face at every moment. :-)

I don't remember that myself, although I know the Greek centaurs were pretty wild and unruly and savage, and were especially dangerous when they got drunk. I've just had a look around at a couple of websites and the only reference I could find to their diet was that they ate raw meat. Which animal the meat came from wasn't specified...

My assumption is that the average centaur would feel as repulsed by the idea of bestiality as a human would be - defining it as "sex with a non-intelligent animal".

Yet more useless trivia from wikipedia: "The Thessalian tribes also claimed their horse breeds were descended from the centaurs[...]", which implies that at least for classical centaurs the bestiality taboo was not as strong as thought.

(Interestingly enough, I don't recall ever seeing classical depictions of female centaurs, so that might have been the standard way of mating...)